[Refactor] Extract GetDisplayAvatarUrl in IUser (#2771)

* Implement `GetDisplayAvatarUrl` method

* Remove obsolete example

* Update XML documentation

---------

Co-authored-by: Quin Lynch <49576606+quinchs@users.noreply.github.com>

src/Discord.Net.Rest/Entities/Users/RestGuildUser.cs

@@ -194,15 +194,13 @@ namespace Discord.Rest
             return new ChannelPermissions(Permissions.ResolveChannel(Guild, this, channel, guildPerms.RawValue));
         }
 
-        /// <inheritdoc />
-        public string GetDisplayAvatarUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
-            => GuildAvatarId is not null
-                ? GetGuildAvatarUrl(format, size)
-                : GetAvatarUrl(format, size);
-
         /// <inheritdoc />
         public string GetGuildAvatarUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
             => CDN.GetGuildUserAvatarUrl(Id, GuildId, GuildAvatarId, size, format);
+
+        /// <inheritdoc />
+        public override string GetDisplayAvatarUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
+            => GetGuildAvatarUrl(format, size) ?? base.GetDisplayAvatarUrl(format, size);
         #endregion
 
         #region IGuildUser

src/Discord.Net.Rest/Entities/Users/RestUser.cs

@@ -142,6 +142,9 @@ namespace Discord.Rest
                 ? CDN.GetDefaultUserAvatarUrl(DiscriminatorValue)
                 : CDN.GetDefaultUserAvatarUrl(Id);
 
+        public virtual string GetDisplayAvatarUrl(ImageFormat format = ImageFormat.Auto, ushort size = 128)
+            => GetAvatarUrl(format, size) ?? GetDefaultAvatarUrl(); 
+            
         /// <inheritdoc />
         public string GetAvatarDecorationUrl()
             => AvatarDecorationHash is not null

src/Discord.Net.Rest/Entities/Users/RestWebhookUser.cs

@@ -60,8 +60,6 @@ namespace Discord.Rest
         /// <inheritdoc />
         string IGuildUser.GuildAvatarId => null;
         /// <inheritdoc />
-        string IGuildUser.GetDisplayAvatarUrl(ImageFormat format, ushort size) => null;
-        /// <inheritdoc />
         string IGuildUser.GetGuildAvatarUrl(ImageFormat format, ushort size) => null;
         /// <inheritdoc />
         bool? IGuildUser.IsPending => null;